This invention relates to dynamoelectric machines such as synchronous motors for operation from an AC power supply of six or more phases with fractional-slot windings that are slightly unbalanced but still achieve low torque harmonics.
Static power conversion equipment is used to convert three phase power from a power line to a higher number of phases for driving motors. For example, adjustable frequency synchronous motors are being designed for six, twelve and twenty-four pulse inverter systems for three, six and 12 phase operation, respectively. Even higher pulse numbers may be used in the future. The use of pulse numbers of multiples of six is desirable to reduce harmonic torque levels and damper losses. The design of such motors is complicated by the fact that a motor manufacturer has available certain existing stator punchings which fix the number of available slots. The available punchings and coil designs are primarily designed for six pulse (three phase) operation and often are not readily adaptable to six or 12 phase machines. A general purpose of this invention is to provide machines with windings for multiples of three phases in cores having slot designs for three phases and to do so without adversely affecting machine performance to any significant degree.
It is straightforward to take an existing machine design for three phase operation and to simply regroup the windings to provide six phase operation if the slot numbers happen to be correct for the higher number of phases. For example, a six pole, 72 slot machine offers a variety of possibilities. It can be connected without problems for six pulse, 12 pulse, or 24 pulse operation, that is either three, six or 12 phases, because the number of coil slots per phase per pole is in each instant an integer. This is another way of saying that the winding for the number of slots used for three phases can be regrouped into two or four separate windings for six or 12 phases. This permits both six phase and 12 phase balanced windings, with uniform spacing, easily.
Uniform spacing is desirable so that adjacent voltage peaks produced by uniform converter pulse spacing are separated by 360.degree. divided by the pulse number. Therefore, the ideal angle for six phase (12 pulse) operation is 30.degree. and for 12 phase (24 pulse) operation it is 15.degree.. Unless the spacing conditions are satisfactory, a principal reason for going to pulse numbers higher than six, namely to achieve reduced torque harmonics, would be defeated.
The problem is that sometimes the number of slots per phase per pole of the desired machine is not an integer. For example, a three phase, six pole machine with 90 slots has five slots per phase per pole but if it is desired to use that same slot number for a six phase, six pole machine the number of slots per phase per pole is 21/2 and for a twelve phase machine the number of slots per phase per pole is 11/4. This means a normal winding is not attainable for a six or 12 phase machine of six poles and 90 slots. Some designers might be tempted to circumvent the problem by using balanced windings and uniform spacing but allowing the slot numbers to be unequal. This type of winding is referred to as a balanced, fractional-slot winding. Such a winding for the six phase embodiment would use three slots per phase per pole for a first winding (for phases A, -C and B) and two slots per phase per pole for a second winding (for phases A', -C' and B'). This would result in voltages of 4800 volts and 3200 volts, respectively, in the two windings in a given application instead of the desired voltage of 4000 volts for both windings and would have adverse effects on the equipment driven by the motor.
The art of winding three phase AC machines (for example, see Liwschitz-Garik, Winding Alternating-Current Machines, D. Van Nostrand Company, 1950, which is a standard reference) includes both balanced fractional-slot windings and unbalanced fractional-slot windings. What is not believed previously known is a design technique for multiples of three phases with unbalanced fractional-slot windings that achieves objectives of low harmonics and nearly equal voltages.
In accordance with the present invention, unbalanced fractional-slot windings are used for six phases, or other multiples of three phases, with a winding connection arrangement such that substantially equal voltages are provided and torque harmonics are minimized. In general, the winding scheme of this invention applies to machines in which the number of coil slots per phase per pole is a non-integer expressible as a simple fraction N/D where D is two or more and the number of phases is at least six and is an integral multiple of three. The winding comprises a plurality of coils of which selected numbers are connected into coil groups of unequal numbers of coils distributed throughout the machine for a particular phase. The numbers of coils within the coil groups is related to the ratio N/D. The number of successive pole windings whose coil groups for the total number of phases is a distinct repeatable pattern for the total winding is equal to D. The number of individual coils within the coil groups for each individual phase within the D pole windings is N.
Several examples of machine arrangements in accordance with the present invention are given hereinafter and it will be apparent how the technique of the invention may be applied to other specific embodiments. The present invention provides a degree of design flexibility not previously available because now a winding design can be provided that achieves the purposes of accommodating an existing core pattern that has been primarily designed for use for three phase windings but is now extended to applications of multiples of three phase windings with the achievement of relatively balanced voltages and reduced harmonics by the selective application of unbalanced windings.